On the resumption Howarth missed with two efforts before a superb kick from the half way by Nathan Spooner gave Leinster the lead once more and deep in injury time Spooner who looked out of sorts in the opening half, kicked a second mighty effort to put Leinster six points up and they held on for a famous victory.
Spooner had put kicked a first half penalty and converted Eric Miller's try but they were clearly second best in that half and tries from Ben Breeze and Andy Marinos gave the home side a deserved interval lead.
The match turned when McKenna robbed Matt Pini to get that run way try. But Leinster still had a lot of hard work to do before they achieved a marvellous victory.
Leinster. G Dempsey; D Hickie, B O'Driscoll, S Horgan, G D'Arcy; N Spooner, B O'Meara; R Corrigan (capt), S Byrne, E Byrne, L Cullen, M O'Kelly, E Miller, V Costello, K Gleeson. Replacements: P Coyle, G Hickie, B Casey, T Brennan, B Willis, A Dunne, A Magro.
Referee: S Dickson (Scotland